Negative stiffness can be generated by non-linear behavior. For instance, simple and widely used non-linear structures that can generate negative stiffness include snap-through beams, buckling beams, over-rotation, and rolling or sliding contact between components. Non-linear structures that exhibit both positive and negative stiffness are potentially useful in a variety of mechanical design applications. For instance, a negative stiffness element (e.g., a buckling-type beam that can exhibit non-linear behavior) can be combined with a positive stiffness element, such as a supporting spring, to provide a structure having zero or quasi-zero stiffness (QZS) over a range of displacements. The quasi-zero stiffness of the structure may be used to isolate another object or mass (e.g. a structure, device, package, and/or an instrument) from unwanted vibrations because the transmission of vibrations through systems of very low stiffness is minimal. However, these conventional isolating structures tend to be unstable in their isolating mode and are more easily utilized for their damping and shock isolation abilities, because manufacturing technologies and techniques are typically not accurate enough to create a QZS structure that does not require frequent tuning. Additionally, conventional QZS structures may not be capable of maintaining low to zero stiffness in the presence of large static load changes (e.g., large static force changes), which may induce a large displacement in the system. One approach is to use a motor to adjust a secondary, softer positive spring in parallel to the main support spring. However, this approach has the limitation of only being able to adjust to small changes in force. Another approach uses a passive negative stiffness system where a large motion would cause the negative stiffness element to disengage and slip in relation to the positive element. However, this system has a disadvantage that it is unable to reset to its minimum stiffness after slippage, which limits its ability to isolate unwanted vibrations.
Accordingly, what is desired is a method of re-centering negative stiffness without completely disengaging the negative stiffness element.